1. Field of the Invention
This invention relates generally to a method and apparatus for detection of analytes, and more particularly to such method and apparatus for the painless detection of such analytes in extracellular space.
2. Description of the Prior Art
The past decade has seen an increased effort for new and less invasive means of detecting relevant compounds in biological systems. In the past such detection has involved clinical assays using in vitro diagnostics requiring invasive sampling methods. Newer philosophies in medicine and related disciplines have tended to use less invasive means that are more comfortable to the subject and which tend to reduce the possibility of transmission of infection.
It is known in the art to use non-invasive spectral techniques to quantitatively determine the presence and concentration of analytes. For example, U.S. Pat. No. 4,882,492 to Schlager teaches a non-invasive near-infrared measurement of blood analyte concentrations, in particular the measurement of blood glucose levels. Schlager recognizes that certain wavelengths of light in the near-infrared spectrum are absorbed by glucose. In that patent, modulated light is directed against a tissue (such as an earlobe). Some portion of the light is passed through the tissue and its spectrum is modified in response to the amount of glucose in the blood and tissue. Similar approaches are shown in U.S. Pat. No. 4,014,321 to March, U.S. Pat. No. 4,655,225 to Dahne et al., U.S. Pat. No. 4,805,623 to Jobsis, and U.S. Pat. No. 5,179,951 to Knudson. However methods using various parts of the infrared spectrum for detecting some analytes are relatively unproven to date due to their inability to differentiate multiple occurrences of the analytes at different concentration levels in different body compartments using the available spectral data.
Cygnus, Inc. has demonstrated a technique for collection of glucose in interstitial fluid found in extra cellular space. This technique uses reverse ionopherisis to attract heavier salt molecules to the outside surface of the skin and uses electrochemical means for detecting glucose. This method is currently the subject of an FDA PMA review. One drawback of this method is the initial time delay for retrieval of sufficient fluid for sampling. This method also requires the subject to constantly have a device in place for collection of the fluid.
SpectRx has developed an alternative method for accessing the interstitial fluid by using a laser to ablate small apertures in the surface of the skin sufficient to release interstitial fluid from extra cellular space. Although this method has also produced data which correlates well with constituents found in blood, it suffers from a progressive closure of the aperture and a correlation between sufficient aperture size and the amount of discomfort caused by the initial ablation of the skin.
TCPI has developed a method similar to that of Cygnus which draws interstitial fluid outside the skin. This method however uses a replaceable patch that changes colormetrically according to the presence of the analyte. This method also requires several minutes more than present blood testing methods.
Integ has disclosed a method to access interstitial fluid in an intradermal layer, sampling the interstitial fluid by drawing interstitial fluid through a capillary and then testing the fluid in situ or extracting the fluid to an external apparatus. The Integ method may be prone to rupture of small capillaries that are found in the upper layers of the dermis and to contact with nerve endings that abut the dermal/epidermal junction. There has also been controversial data produced as to whether glucose levels in particular track well between levels produced in the intradermal interstitial fluid and that found in capillaries or veins. Apparently the Integ method draws a one microliter sample from the interstitial fluid, which sample is analyzed for glucose.
In summary, the prior art methods could be improved in that they suffer from insufficient sampling quantities, delayed periods of testing required for sample collection, destruction of skin cells at the stratum corneum level, discomfort to the subject, and the production of quantities of blood.